Re: Detecting page cache trashing state

From: Daniel Walker
Date: Fri Sep 15 2017 - 13:31:35 EST

On 09/15/2017 09:38 AM, Taras Kondratiuk wrote:
Quoting Daniel Walker (2017-09-15 07:22:27)
On 09/14/2017 05:16 PM, Taras Kondratiuk wrote:

In our devices under low memory conditions we often get into a trashing
state when system spends most of the time re-reading pages of .text
sections from a file system (squashfs in our case). Working set doesn't
fit into available page cache, so it is expected. The issue is that
OOM killer doesn't get triggered because there is still memory for
reclaiming. System may stuck in this state for a quite some time and
usually dies because of watchdogs.

We are trying to detect such trashing state early to take some
preventive actions. It should be a pretty common issue, but for now we
haven't find any existing VM/IO statistics that can reliably detect such

Most of metrics provide absolute values: number/rate of page faults,
rate of IO operations, number of stolen pages, etc. For a specific
device configuration we can determine threshold values for those
parameters that will detect trashing state, but it is not feasible for
hundreds of device configurations.

We are looking for some relative metric like "percent of CPU time spent
handling major page faults". With such relative metric we could use a
common threshold across all devices. For now we have added such metric
to /proc/stat in our kernel, but we would like to find some mechanism
available in upstream kernel.

Has somebody faced similar issue? How are you solving it?

Did you make any attempt to tune swappiness ?



This control is used to define how aggressive the kernel will swap
memory pages. Higher values will increase agressiveness, lower values
decrease the amount of swap.

The default value is 60.

Since your using squashfs I would guess that's going to act like swap.
The default tune of 60 is most likely for x86 servers which may not be a
good value for some other device.
Swap is disabled in our systems, so anonymous pages can't be evicted.
As per my understanding swappiness tune is irrelevant.

Even with enabled swap swappiness tune can't help much in this case. If
working set doesn't fit into available page cache we will hit the same
trashing state.

I think it's our lack of understanding of how the VM works. If the system has no swap, then the system shouldn't start evicting pages unless you have %100 memory utilization, then the only place for those pages to go is back into the backing store, squashfs in this case.

What your suggesting is that there is still free memory, which means something must be evicting page more aggressively then waiting till %100 utilization. Maybe someone more knownlegable about the VM subsystem can clear this up.